Modified Bitumen Roofing and Telltale Signs – Part III of VII

Last week we started a new series on modified bitumen roofing, a very common type of flat roof membrane here in Capitol Hill and Washington dc. Our company normally installs TPO roofing on full new roof jobs, even on old existing buildings.  Since modified bitumen roof materials are one of the most common types of low slope roofing used in our area though, our company has to be very familiar with the this type of roof membrane so we know how to repair it when it’s been installed wrong or when it’s damaged or deteriorated. It’s very common that new clients or even our old clients call us out to fix the problems associated with modified bitumen roofing, especially when it’s been initially installed by a different contractor.

The outline for the entirety of this seven-part series on modified bitumen flat roofing follows, with todays portion in bold:

  1. What is Modified Bitumen Roofing?
  2. Hard Requirements 
  3. Best Practices
    1. Cant strips
    2. Strip-in flashing
    3. Consistent Bleed-out
    4. Granules in bleed-out
    5. Primer at metal terminations
    6. Liberal seal at application
    7. Coatings AFTER installation 
  4. Telltale Sign Of Experience
  5. Core Sampling 
  6. Rounded Corners and Why They are Better
  7. Bleed-out as a Sign for Proper Application 

Today, we will talk about some of the best practices for installation of both modified bitumen roofing and other types of low slope or flat roofing.  

Cant strips

Cant strips are angled buildup elements, similar to wood but often made from a lightweight cellulose type of material, in most cases, which change the contour of the low slope or flat roof deck. Where the flat deck meets the parapet walls, there’s generally a 90° angle. For modified bitumen membranes, when installed over a 90° angle, that angle is too sharp or too extreme for the membrane to adapt.  The sharp angle creates a point of stress in the membrane. Cant strips decrease this angle making it easier for the modified bitumen membrane to adapt.  

It’s unfortunately a bit rare for modified bitumen membrane installers, in the Capitol Hill in Washington DC area, to install cant strips properly as required by the manufacturers and therefore by the building code in most cases. In large scale commercial construction proper standards are often followed because more professional companies are doing the work. Our company, for example, is a professional company that is trained and experienced in the principles of flat roofing. We are however rare in this market niche.

strip flashing
A granulated modified bitumen membrane haphazardly installed up a parapet wall and over the top of the parapet without coping which is required by the building code.

Strip-in flashing

Strip-in flashing is used at termination such as gravel stops and drip edges. These types of terminations are generally pre-bent or fabricated with light gauge sheet metal. When they are bent they are formed into slightly more rigid shapes that allow the roof membrane to terminate in a secure manner. The roof must be properly terminated so that it ends in a way that is more secure than just being adhered to the substrate. In the field of the roof, for example, it’s totally fine for the membrane to be just adhered to the substrate because all parts of that membrane in the field are attached to other parts of the membrane in the field. At a termination though, there is an otherwise open edge which cannot withstand the stress and force of high winds and weather. The metal terminations such as the drip edges or gravel stops allow that roof membrane to me terminated in a secure way.

strip in flashing

There’s nothing unusual about seeing terminations such as gravel stops or drip edges, but not all terminations are properly installed with strip in flashing. Many roofers take a shortcut by just installing the membrane on top of the termination. Some membranes though are required to be installed under the metal termination, to be secured with fasteners.  Then, once the metal termination is installed a new strip of membrane will be installed on top of the metal termination and it will span from the metal termination back onto the substrate main roof membrane.

Consistent Bleed-out

Consistent bleed-outs are one of the most important signs that a roofing installer, especially for modified bitumen roofing, is experienced and comfortable using a torch to adhere the membrane to the substrate. When the modified bitumen membrane is heated with a torch, the emulsion on the underside of the membrane is slightly melted. That emulsion then becomes tacky and can adhere the roll membrane to the roof substrate. There are two main families or classifications of roofing in the modified bitumen category. There is a type of modified bitumen membrane called SBS and another type called APP.  For SBS membranes, for example, you generally expect to see between 3/8 of an inch to 1 and 3/8 of an inch of bleed-out, in this case is the amount of the modified bitumen emulsion that bleeds out from underneath the membrane to the exposed lower edge.

When you see bleed-out that exceeds one and three of an inch, it’s a sign that the membrane was overheated and overheating of the membrane can cause deterioration or debilitation of the structure of the supporting ply and fiber elements of the membrane.  Conversely, when bleed out is less than 3/8 of an inch, it likely was not properly heated with enough heat to allow for good adhesion to the substrate. These are important signs of quality.

bleed out
Here a modbit membrane has sufficient bleed-out but the membrane has never been coated as required and the surface now has an alligator skin.

Upkeep, Care and Maintenance of Washington DC Roofs

This case study is a perfect example of why a professional roofer should be used for all types of roof installation, even things that seem as simple as a balcony.  Consulting with a roofing professional, like Dupont Roofing, familiar with the specific climate conditions and the building science of roofing in Washington DC is advisable to ensure the roof assembly is appropriately designed and constructed to manage the flow of water, ice damming, capillary action and condensation effectively.

The upkeep, maintenance, and general care of flat and mansard roof systems should be driven by an understanding and passion for historical methodologies, waterproofing principles, engineering and building science. Washington DC, a city built with both vintage charm and contemporary modernities, residential and commercial buildings of substantial value. 

The roofs of these buildings are their defensive shield from the harsh elements of nature. To our clients, as well as all readers of this article and our blog, we emphasize the importance of quality construction and active building maintenance.  Our website includes informative resources you can use to understand and learn about best practices on preservation of your building.  If you are in need of further guidance on the roof and its associated systems for your Washington DC property, we are here to assist, where we can. Simply complete the webform below and drop us a line and we’ll respond if we can help.

On Key

Related Posts

what damages metal roof paint

Terne Metal Roof Paint Deterioration

Terne-coated metal roofs are an element of historic architecture, somewhat common in Capitol Hill and other historic neighborhoods of Washington, DC. However, the terne coating